The present invention relates to a relatively mild thermal-extraction process for converting the organic material of oil-shale to predominantly liquids. The oil-shale is first heated to a temperature from about 360.degree. C. to 475.degree. C. in an inert atmosphere. The resulting liquids and gases are collected and the residue is extracted with a microemulsion to recover additional amounts of liquids.
Oil-shale, one of the leading sources under investigation for the production of synthetic fuels, may play a leading role in the energy future of the United States. The primary reason for the growing importance of oil-shale, as well as coal, is the rapid depletion of known petroleum and natural gas reserves. These known reserves are being depleted at a faster rate than the rate of discovering new reserves. As the era of petroleum growth draws to a close, the worlds energy mix will have to change. Transition energy sources will be needed as a bridge between petroleum and the potentially unlimited energy sources of the future; such sources being, for example, solar power and nuclear fusion. Owing to their great abundance, coal and oil-shale are perceived as the keystones of such a bridge. Consequently, a great deal of research and development is presently in progress to provide economical ways of converting these solid resources to valuable liquids and gases.
Generally, oil is produced from oil-shale by heating the oil-shale in either a fixed or moving bed reactor at a temperature from about 500.degree. to 550.degree. C. for a long enough time to convert the organic matter to gas, oil, and coke. The volatile conversion products are then conventionally distilled from the inorganic residue, or spent shale. Another technique known in the art for obtaining oil from oil-shale is the heating of oil-shale at elevated temperatures, either alone or in the presence of solvent, followed by extracting the soluble conversion products with an extracting agent. The extraction is an alternative to distillation.
An example of a low temperature process which is conventionally used for producing liquids from oil-shale is the process described in a paper entitled Controlled Low-Temperature Pyrolysis of Benzene-Extracted Green River Oil-Shale by J. J. Cummins and W. E. Robinson from "Preprints of the American Chemical Society Division of Fuel Chemistry", Vol. 12, pp. 41-50, 1968. Such a low temperature process comprises heating the oil-shale in a helium atmosphere at atmospheric pressure, at a temperature from 300.degree. C. to 350.degree. C., for a period of time ranging from 12 to 96 hours.
Another process disclosed in the art is the process described in "Bureau of Mines Report of Investigations 4744", November 1950, entitled A Thermal Decomposition Study of Colorado Oil-Shale by A. B. Hubbard and W. E. Robinson. According to this Bureau of Mines Report, a two step process is not necessary because they believe that heating the oil-shale for an extended period of time in only one step results in the same conversions as if the oil-shale was first heated for a shorter period of time followed by extraction with an extracting agent. This is contrary to the teaching of the present invention.
Still another process taught in the art is the process taught in U.K. Pat. No. 291,475 (1928) which teaches that oil-shale first undergoes preliminary heating at a temperature from 300.degree. C. to 305.degree. C. for 100 to 120 minutes, followed by heating the material further to a temperature of 400.degree. C. to 600.degree. C., then followed by extracting the oil with an entraining agent which may be an oil fraction from the oil-shale. It will be shown by the examples to follow that subjecting the oil-shale to a relatively low-temperature preheat is detrimental for obtaining optimum yields of liquid product.
Even though myriad processes for obtaining oil from oil-shale are disclosed in literally thousands of publications and patents published over the last hundred years, there still exists a need in the art for a better scientific understanding of oil-shale as well as processes which would make the commercialization of oil from oil-shale more economical.